Addressing paradoxes with Weapons designed to hit a target in the past

Question

In space, normal weapons are so easy to dodge because by the time you see an enemy ship and fire at it, it's already moved out of the way. Now imagine a weapon that travels backwards in time as it flies though space so that when it arrives at its target, it converges with the exact time and location that you saw the target when you fired. So, if an enemy ship is 5 light minutes away. Then your shot travels back in time 5 minutes as it closes the distance eliminating all guess work about where the ship will be.

For purposes of this question, assume this weapon can only travel back in time as fast as light can travel forward in time. So you can target a thing in the past exactly where you see it now, but you cant target something you saw 10 minutes ago.

While such a weapon would seem to work without any major paradoxes when you just have one ship shooting another, what would happen if 2 ships shot at each other with such a weapon. Since both ships could in theory be destroyed before either ship actually fires thier weapons does there need to be some rule that one event will take precedence over the other, or is there a logically consistent way for both ships to destroy each other in this manner since neither ship captain could see any future events that might cause him to change his course of action.

Answer 1

Frame challenge

A time-travel paradox is by definition an inconsistency that cannot be resolved. This is why people think it's so important to avoid them: because it is logically impossible to resolve a paradox if one arises.

Thus, there is no "logically consistent" way for this to happen. You are going to have to invent some new rule that defies ordinary logic. As the author, you can do this! As the author, you cannot decide whether your audience will buy it.

We like to think that one of the best ways to ensure that audiences will accept our narratives is if we stick within the bounds of conventional reason. But works like Star Wars and Star Trek do all kinds of things that are patently absurd, and that has not stopped countless people from enjoying them.

Answer 2

there is no Paradox

As written in the question

Then your shot travels back in time 5 minutes as it closes the distance

the shot doesn't begin its trajectory 5 min back and then travels to the enemy. Instead it travels back in time by moving toward the enemy.

So i agree with ivo's comment and mostly with JBH' answer except for his proposed paradox.

Because the shot isn't send back in time but travels backwards while moving there is no way to detect it before it hits. It would just seem to materialize into the enemy ship.

It seems to me that this is just a "teleportation weapon" in effect. You press a button, and in that instant your projectile hits. No time to dodge or even realize something is closing in, so no chance for dodging and the resulting paradox. (To be fair this is a jab at the other proposed answers, because they use it as an example of the loop that can result when the shot is send back in time and then travels toward the enemy.)

On a timeline it would seem to work like this:

T0 You spot the enemy.
T0 You fire and see the enemy getting hit.

In full:

T-5 The enemy reflects the light you see at T0, your projectile hits.
T-4 Your shot has traversed 80% of the distance and moved 4 min backwards in time.
T-3 Your shot has traversed 60% of the distance and moved 3 min backwards in time.
T-2 Your shot has traversed 40% of the distance and moved 2 min backwards in time.
T-1 Your shot has traversed 20% of the distance and moved 1 min backwards in time.
T0 You spot the enemy.
T0 You fire and see the enemy getting hit.

Answer 3

Whoever hits first gets to control the timeline.

If ship A hits ship B first, then ship B can't fire at ship A. Therefore, their temporal projectile vanishes, and they do no harm.

Example.

1. Ship A's projectile goes back in time and Ship A and B fly unaware.
2. Ship A and Ship B see each other.
3. Both ships prepare to fire back in time, but Ship A fires first.
4. Ship B explodes immediately from the back in time projectile.

Answer 4

What paradox?

All ships are 100% automatic (it doesn't matter if it they are or not, it's just easier for the explanation). The moment my ship detects the enemy ship, it fires the weapon. The very moment the circuits activate the weapon, my enemy is hit.

Ditto for my enemy, which did the same to me.

Where's the paradox? If my ship detected the enemy ship first, the enemy ship won't get a shot off. If the enemy ship detected my ship first, I won't get a shot off. If we detected each other simultaneously, neither of us will be around to argue about whether or not there was a paradox.

But I want a rule!

I literally can think of only one scenario where a paradox can occur. Your detonation can occur before the weapon is activated. In that case, the paradox of your ship destroying another ship, but you were destroyed before you activated your weapon, could occur.

Rule: don't do that. Regardless the range you wish to give your weapon, no detonation can occur before the weapon is activated. In other words, time travel in your universe is causal. No paradox can occur because no effect can occur without the cause that brought the effect about.

And why do you want your rule? Because if you didn't then all it would take is one drunken sailor ordered to fire the weapon to look at his friend and say, "watch, this will be funny" and not push the button to rip all of space and time apart. You don't want that. Nobody wants that. Well, psychopaths might want that. But we're not psychopaths, right? RIGHT?

Answer 5

No real paradox free time travel

Unless the time travelling component is required, you can get around this by making weapons fire their projectiles at very near light speed. You could say they somehow weaponized neutrinos or something similar. This would have very similar results in that the attacks are unable to be dodged, undetectable, and can only hit a target where they are currently at (unless they are extremely far away).

Another good way to deal with this is treating the shots as instantly teleporting the distance as Mileonen mentioned. While this does result in problems of FTL travel, those are a lot easier to ignore than time travel.

The paradox you proposed of not having a ship to shoot at so why shoot does happen anyway regardless of whether there are the two ships are firing at each other. If the ship is destroyed before you would have pushed the button to fire, then there would be no need to press it in the first place so you wouldn't have fired the shot that destroyed the enemy ship.

If the time travel is important I would follow what JBH recommended and make a rule where time travel is causal.

Another way around this might be that the weapon's computer and projectile exists in its current state across time eg any firing information and the fired projectile exists on it in the past present and future. With the time of the attack being part of the firing information the projectile will only meaningfully "exist" only for the moment it strikes the target. This, along with the firing information only meaningfully existing for the crew when the data is entered and after that, would remove the paradox (or at least make it feel like its not there, there is probably something I am overlooking). This is a lot more complicated though.

Answer 6

Suppose that Novikov's self-consistency principle applies. In this scenario, there are two outcomes that are each self-consistent: ship A destroys ship B before ship B fires, or ship B destroys ship A before ship A fires. Which of these outcomes actually occurs is a matter of random quantum mechanical probability, determined by a sum-over-histories wavefunction result that I suspect may be infeasible to calculate for any semi-realistic scenario at this scale.

Answer 7

I think the paradox can be resolved by adding a necessary delay: mass prevents time travel. The mass of the firing ship means the weapon needs to travel a bit before it can start traveling backwards, and the mass of the target ship likewise causes the weapon to reenter normal time a short distance away. Just tune the effect so the arrival time is too small to allow a response and the weapon will largely function like you want. The mass of the weapon itself isn't relevant, as the whole thing will time travel. The relative masses of the two ships is also not important, as the departure delay for one ship;s weapon will equal the arrival delay for the other ship's weapon, so the total trip is functionally symmetrical in both directions (less massive ships would have a slight advantage in departure time, but an equal disadvantage in arrival time).

So simultaneous shots will both arrive, as the two shots will have time to travel before their source is destroyed. Near simultaneous shots will also result in both ships being destroyed, due to the delays.

Super massive ships (I'm thinking planetoid mass) would have enough arrival delay to respond with point weapons, but the cost to make, move, and maintain such a ship would be prohibitive. Likewise, the time travel feature of the weapon isn't usable too close to a planet, or more likely inside a solar system at all. Maybe outer reaches are low mass enough. Battle fields with asteroids or debris or whatever would require carefully aimed shots to avoid having the weapon knocked out of time travel too soon.

Answer 8

Seems to ME that it's really contingent on the mechanic by which you decide time "actually" functions. Even Einstein and Hawking kinda "settled" into the whole infinite parallel universe thing for lack of better means to reconcile these sort of incongruities.

Fact is, for you to have cause and effect, action/reaction, determinism/consequence, there's ALWAYS a sequence of events. And there's one thing nearly all those theoretical physicists DO agree on: time is relative from the point of view of the observer.

So my suggestion would be reductionism: break the sequence of events down into smaller and smaller chunks and ultimately there's no paradox. Yes, it's possible one party is retroactively "deleted" from the timeline. Whereupon, for the frame of reference of their opponent, they never fired. They ceased before it could become a conflict. And since their opponent, too is submerged in the same stream of time, they, perforce, would be unable to perceive any sort of potential paradox.

Or, take the string theory approach and assuming every choice bifurcates into every possible choice, and obviate the potential for it entirely.

Personally, I think the mechanic by which the characters could have perceived an event taking place that was subsequently balefire'd out of existence yet they still remember it would/could/should be harder to explain than the nitty gritty of the temporal torpedo... but maybe that's just me.

Answer 9

The flight time of the weapon is instantaneous, in effect.

It's just as if the two ships were literally touching nose-to-nose.

It is asked about what happens if "the ships fired at the same time." Same thing happens as would happen were they nose to nose, I suppose. Either "the same time" really means plus or minus a few seconds or milliseconds, and one ship is destroyed before it can fire, or, if it is truly instantaneous, it depends on whether the weapons are such that they can complete firing while being destroyed.

Answer 10

The first major problem is (as defined in your question) that even assuming a time traveling weapon was possible the observer in control of it cannot! The weapon can be fired at a set of coordinates but there exists no targeting system which lets the user observe the past to detect approaching threats! This means that in terms of space combat any two ships randomly encountering each other are limited to the moment one or other vessel first detects it's opponent via the same old boring sensors like radar or thermal imaging etc that everyone else uses in space combat.

You literally can't know in advance using your 'time scope' (because you don't have one) that your enemy will appear at co-ordinates XYZ in precisely 10 minutes time. That means your limited to using conventional sensors to locate and target your opponents and all such sensors have margins for error. So the longer the range the less accurate your targeting is going to be. Which means you precisely the same dilemma you have with conventional weapons. You can either fire at long range with a high probability of missing the target or wait till your targeting solution becomes better.

The second limitation is related to the first in that time equals distance in this context. The farther back in time you attempt to aim the farther away in space the target will be which again means the less accurate your weapon becomes. So its not like you can 'see' and enemy and then switch the weapon to '3 years ago' aim at the space dock where that ship was being built and blow it up before its even launched! And since range increases the farther back in time you try to fire inaccuracy must also. (Except in some particularity rare or unusual combinations e.g. 2 ships traveling close together for years on time who suddenly decide to start shooting 'time bombs' at each other.)